Photovoltaic cell

ABSTRACT

A photovoltaic cell is described, having a photoactive layer ( 4 ) made of two molecular components, namely an electron donor and an electron acceptor, particularly a conjugated polymer component and a fullerene component, and having two metallic electrodes ( 2, 6 ) provided on both sides of the photoactive layer ( 4 ). In order to provide advantageous construction conditions, it is suggested that an intermediate layer ( 5 ) made of a conjugated polymer, which has doping corresponding to the electrode potential and, in regard to the electron energy, has a band gap between the valence band and the conduction band of at least 1.8 eV, be provided between the photoactive layer ( 4 ) and at least one electrode ( 2,6 ).

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of Austrian ApplicationNo. A 735/2000, filed: Apr. 27, 2000. Applicants also claim priorityunder 35 U.S.C. §365 of PCT/AT01/00128, filed: Apr. 27, 2001. Theinternational application under PCT article 21(2) was not published inEnglish.

The present invention relates to a photovoltaic cell having aphotoactive layer made of two components, namely an electron donor andan electron acceptor, particularly a conjugated polymer component and afullerene component, and having two metallic electrodes provided on bothsides of the photoactive layer.

Plastics having extensive π-electron systems, in which single and doublebonds follow one another alternately in sequence, are referred to asconjugated plastics. These conjugated plastics have energy bands whichare comparable with semiconductors in regard to electron energy, so thatthey may also be transferred from the non-conductive state into themetallically conductive state through doping. Examples of suchconjugated plastics are polyphenylenes, polyvinylphenylenes (PPV),polythiophenes, or polyanilines. The efficiency of energy conversion ofphotovoltaic polymer cells made of a conjugated polymer is, however,typically between 10⁻³ and 10⁻²%. To improve this efficiency,heterogeneous layers made of two conjugated polymer components havealready been suggested (U.S. Pat. No. 5,670,791 A), one polymercomponent being used as an electron donor and the other polymercomponent as an electron acceptor. By using fullerenes, particularlybuckminsterfullerenes C₆₀, as electron acceptors (U.S. Pat. No.5,454,880 A), the charge carrier recombination otherwise typical in thephotoactive layer may be largely avoided, which leads to a significantincrease in efficiency. For reaching a good efficiency, good chargeseparation is a necessary, but not sufficient condition, because it mustalso be ensured that the separated charges also reach the correspondingelectrodes of the photovoltaic cell. In typical photovoltaic cells ofthis type, a hole-collecting electrode made of indium/tin oxide (ITO)and an electrode-collecting electron made of aluminum have proventhemselves.

The present invention is therefore based on the object of designing aphotovoltaic cell of the type initially described in such a way that thecharge transport between the photoactive layer and electrodes may beincreased to elevate the short-circuit current.

The present invention achieves the object described in that anintermediate layer made of a conjugated polymer, which has a dopingcorresponding to the electrode potential and, in regard to the electronenergy, has a band gap between the valence band and the conduction bandof at least 1.8 eV, is provided between the photoactive layer and atleast one electrode.

Since the conjugated polymer of the intermediate layer is dopedaccording to the electrode potential, which means an oxidative doping inthe region of the hole-collecting electrode and a reductive doping inthe region of the electron-collecting electrode, the conjugated polymerensures a hole excess in the region of the hole-collecting electrode,but an electron excess in the region of the electron-collectingelectrode, so that, in the region of the oxidatively doped polymer, thehole conduction is reinforced, and the electron conduction is reinforcedin the region of a reductively doped polymer. Since, however, theconjugated polymer of the respective intermediate layer has, in regardto the electron energy bands, a comparatively large band gap of at least1.8 eV between the valence band and the conduction band, acorrespondingly high activation energy results for the intrinsicconduction, which leads, in the case of an oxidatively doped polymerlayer, to the electron conduction from the photoactive layer to thehole-collecting electrode being obstructed and, in the case of thereductively doped intermediate layer, to the hole conduction from thephotoactive layer to the electron-collecting electrode being obstructed.With the aid of these separate intermediate layers, a valve effect maytherefore be achieved, which supports the conduction of the chargecarriers of the respective adjoining electrode which are to be collectedfrom the photoactive layer to the electrode, but obstructs diffusion ofopposing charges in the same direction. As a result of these separatelayers, the charge conduction to the electrodes may correspondingly beimproved, which directly results in an increase of the short-circuitcurrent. It probably does not have to be emphasized that, depending onthe application, photovoltaic cells may be used which have such anintermediate layer between the hole-collecting electrode and thephotoactive layer, between the electron-collecting electrode and thephotoactive layer, or in the region of both electrodes.

Although different conjugated polymers may be accordingly oxidatively orreductively doped to implement the intermediate layers, particularlyadvantageous ratios result if the intermediate layer is made of a dopedpolythiophene derivate. Both high polymers and oligomers are to beunderstood from the concept of polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the object of the present invention is illustrated in thedrawing.

FIG. 1 shows a detail of a photovoltaic cell according to the presentinvention in a schematic section, and

FIG. 2 shows the current-voltage characteristics of a typicalphotovoltaic cell and a photovoltaic cell according to the presentinvention.

As shown in FIG. 1, the photovoltaic cell comprises a transparent glasscarrier 1, onto which an electrode layer 2 made of indium/tin oxide(ITO) is applied. This electrode layer 2 generally has a comparativelyrough surface structure, so that it is covered with a smoothing layer 3made of a polymer, typically PEDOT, which is made electricallyconductive through doping. In contrast to typical photovoltaic cells ofthis type, according to the present invention, photoactive layer 4 isnot applied directly onto the smoothing layer, but rather onto anintermediate layer 5, which is made of conjugated polymer, preferablyfrom a poly(3-alkylthiophene), which is oxidatively doped usingnitrosonium tetrafluoroborate after being applied onto smoothing layer3, in order to obtain a corresponding hole excess.

Photoactive layer 4, which is applied onto intermediate layer 5 in theform of a solution, is made of a conjugated polymer, preferably apolythiophene derivate, as an electron donor, and a fullerene,particularly functionalized fullerene PCBM, as an electron acceptor. Theconcept of polymer is to be understood to mean both high polymers andoligomers. Electron-collecting electrode 6 is made of aluminum, which,in the case of the exemplary embodiment illustrated, is vapor depositedonto photoactive layer 4 without interposing a further intermediatelayer, which would, however, be completely possible. In this case, theconjugated polymer of the intermediate layer would be reductively dopedcorresponding to the negative potential of electron-collecting electrode6, in order to ensure a corresponding electron excess.

Due to the intermediate layer 5, which has a band gap of at least 1.8 eVbetween the valence band and the conduction band, the entrance ofelectrons from photoactive layer 4 into intermediate layer 5 is mademore difficult due to this comparatively broad band gap, without thehole conduction between photoactive layer 4 and hole-collectingelectrode 2 being impaired. The conduction band of the conjugatedpolymer of intermediate layer 5 is, for example, in contrast to PEDOTlayer 3, at a significantly higher energy level than the energy band ofthe electron acceptor of photoactive layer 4. This leads to a unipolarcharge transfer from photoactive layer 4 to hole-collecting electrode 2,which is observable in a corresponding increase of the short-circuitcurrent, as may be seen in FIG. 2. In FIG. 2, current density I isgraphed over voltage U at an excitation energy of 80 mW/cm² undersimulated AM 1.5 conditions for a photovoltaic cell according to thepresent invention in comparison to a cell constructed correspondingly,with the exception of intermediate layer 5. It is shown thatcharacteristic 7, assigned to the photovoltaic cell according to thepresent invention, results in a short-circuit current, measured atvoltage U=0 V, which is approximately twice as large as theshort-circuit current of the comparison cell shown in characteristic 8.

It probably does not have to be noted in more detail that, ifintermediate layer 5 is positioned between photoactive layer 4 andelectron-collecting electrode 6, the distance of the valence bands, andnot of the conduction bands, is decisive.

Since the effect of electrically insulating transition layer 6 is notrestricted to conjugated polymers as electron donors and fullerenes aselectron acceptors, the effect according to the present invention mayalso be observed in all photovoltaic cells having a moleculartwo-component layer made of an electron donor and an electron acceptor.

What is claimed is:
 1. A photovoltaic cell, comprising: a firstelectrode; a second electrode; a first layer between the first andsecond electrodes, the first layer comprising: a first polymer, thefirst polymer being conjugated; and a fullerene component; and a secondlayer between the first electrode and the first layer, the second layercomprising doped poly(3-alkylthiophene).
 2. The photovoltaic cell ofclaim 1, wherein the first polymer comprises a polythiophene derivative.3. The photovoltaic cell of claim 2, wherein the fullerene componentcomprises functionalized fullerene PCBM.
 4. The photovoltaic cell ofclaim 1, wherein the fullerene component comprises functionalizedfullerene PCBM.
 5. The photovoltaic cell of claim 1, wherein the dopedpoly(3-alkylthiophene) has a valence band, a conduction band and a bandgap of at least about 1.8 eV between the valence band and the conductionband.
 6. The photovoltaic cell of claim 1, further comprising a thirdlayer between the first electrode and the second layer, the third layercomprising a third polymer.
 7. The photovoltaic cell of claim 6, whereinthe third polymer comprises PEDOT.
 8. The photovoltaic cell of claim 1,wherein the first electrode comprises ITO and the second electrodecomprises aluminum.
 9. The photovoltaic cell of claim 2, wherein thedoped poly(3-alkylthiophene)has a valence band, a conduction band, and aband gap of at least about 1.8 eV between the valence band and theconduction band.
 10. The photovoltaic cell of claim 2, wherein the firstelectrode comprises ITO and the second electrode comprises aluminum. 11.A photovoltaic cell, comprising: a first electrode; a second electrode;a first layer between the first and second electrodes, the first layercomprising: a polythiophene derivative; and a fullerene component; asecond layer between the first electrode and the first layer, the secondlayer comprising a doped polythiophene derivative; and a third layerbetween the first electrode and the second layer, the third layercomprising a third polymer.
 12. The photovoltaic cell of claim 11,wherein the fullerene component comprises functionalized fullerene PCBM.13. The photovoltaic cell of claim 11, wherein the doped polythiophenederivative comprises doped poly(3-alkylthiophene).
 14. The photovoltaiccell of claim 13, wherein the doped poly(3-alkylthiophene) has a valenceband, a conduction band, and a band gap of at least about 1.8 eV betweenthe valence band and the conduction band.
 15. The photovoltaic cell ofclaim 11, wherein the doped polythiophene derivative has a valence band,a conduction band and a band gap of at least about 1.8 eV between thevalence band and the conduction band.
 16. The photovoltaic cell of claim11, wherein the third layer comprises PEDOT.
 17. The photovoltaic cellof claim 11, wherein the first electrode comprises ITO and the secondelectrode comprises aluminum.
 18. A photovoltaic cell, comprising: afirst electrode; a second electrode; a first layer between the first andsecond electrodes, the first layer comprising: a first polymer, thefirst polymer being conjugated; and a fullerene component; and a secondlayer between the first electrode and the first layer, the second layercomprising a second polymer, the second polymer being a doped conjugatedpolymer; and a third layer between the first electrode and the secondlayer, the third layer comprising a third polymer.
 19. The photovoltaiccell of claim 18, wherein the first polymer comprises a polythiophenederivative.
 20. The photovoltaic cell of claim 18, wherein the fullerenecomponent comprises functionalized fullerene PCBM.
 21. The photovoltaiccell of claim 18, wherein the second polymer has a valence band, aconduction band, and a band gap of at least about 1.8 eV between thevalence band and the conduction band.
 22. The photovoltaic cell of claim18, wherein the third polymer comprises PEDOT.
 23. The photovoltaic cellof claim 18, wherein the first electrode comprises ITO and the secondelectrode comprises aluminum.